Conventionally, 340 MPa class bake-hardenable (BH) steel sheets (hereinafter referred to as “340BH”) have been applied to automotive outer panels such as hoods, doors, trunk lids, back doors, and fenders, which require dent resistance.
340BH is a ferrite single-phase steel produced by adding carbide or nitride-forming elements such as niobium and titanium to an ultralow carbon steel containing less than 0.01% by mass of carbon to control the amount of carbon dissolved therein and strengthening the steel with manganese and phosphorus by solid solution strengthening. There has been a growing need for lightweight car bodies. Further research has been conducted on, for example, further increasing the strength of outer panels, to which 340BH has been applied, to reduce the thickness of the steel sheets, reducing the number of reinforcements (R/F; inner reinforcing parts) with the same thickness, and reducing the temperature and time of a bake hardening process.
However, adding larger amounts of manganese and phosphorus to the conventional 340BH for increased strength noticeably degrades the surface distortion resistance of press-formed products because YP increases. The term “surface distortion” refers to a pattern of extremely small wrinkles and waves that tend to appear on a press-formed surface, for example, at the periphery of a doorknob.
Surface distortion noticeably impairs the surface appearance quality of automobiles. Therefore, a steel sheet applied to outer panels requires a low YP close to that of the currently used 340BH as well as increased strength of pressed products.
In addition, steels having higher strengths than 340BH tend to have variations in material properties such as YP, TS, and El, and are therefore liable to surface distortion and breakage. If such steel sheets with high YP have little variation in material properties, surface distortion on design surfaces can be reduced by adjusting the shape of a press die. However, it is extremely difficult to reduce surface distortion if YP and TS vary within a coil in the longitudinal or width direction, or vary between coils. This is because grinding a press die to adjust the surface shape for each coil is impractical in mass production, and adjusting the press conditions such as forming pressure has a little effect of improving surface distortion. Accordingly, there is a need for a high strength steel sheet having low YP and little variation in material properties within a coil or between coils at the same time.
Furthermore, a steel sheet used for automobiles is also required to have excellent corrosion resistance. Since steel sheets are closely in contact with each other at a hem processing portion and a spot welding peripheral portion of body parts such as a door, a hood and trunk lid, chemical conversion films are difficult to form by electrocoating. Hence, rust is easy to form. In particular, in corner portions at a front side of a hood and a lower side of a door at which water is liable to remain and which are exposed to a wet atmosphere for a long time, holes are frequently generated by rust.
Furthermore, in recent years, car body manufactures have been considering on increasing the hole-forming resistant life to 12 years from a conventional life of 10 years by improving corrosion resistance of car bodies. Hence, a steel sheet must have sufficient corrosion resistance.
Against this backdrop, for example, Japanese Examined Patent Application Publication No. 6-35619 discloses a technique for producing a cold-rolled steel sheet with high elongation by maintaining a steel containing, in percent by weight, 0.10% to 0.45% of carbon, 0.5% to 1.8% of silicon, 0.5% to 3.0% of manganese, and 0.01% to 0.07% of soluble aluminum in the temperature range of 350° C. to 500° C. for 1 to 30 minutes after annealing to form 5% to 10% or more of retained γ.
In addition, Japanese Examined Patent Application Publication No. 62-40405 discloses a method for producing a high strength steel sheet combining low yield stress (YP), high elongation (El), and high bake hardenability (BH) by adjusting the cooling rate, after annealing, of a steel containing, by weight, 0.005% to 0.15% of carbon, 0.3% to 2.0% of manganese, and 0.023% to 0.8% of chromium to form a dual-phase structure composed mainly of ferrite and martensite.
Furthermore, Japanese Patent No. 3969350 discloses a method for producing a high strength steel sheet having excellent bake hardenability and excellent room-temperature anti-aging properties by adding 0.02% to 1.5% of molybdenum to a steel containing, in percent by mass, more than 0.01% to less than 0.03% of carbon, 0.5% to 2.5% of manganese, and 0.0025% or less of boron and controlling the soluble aluminum, nitrogen, boron, and manganese contents to satisfy sol.Al≧9.7×N and B≧1.5×104×(Mn2+1) to form a microstructure composed of ferrite and a low-temperature transformed phase.
Japanese Patent No. 4113036 discloses that a steel sheet having excellent anti-aging properties at room temperature and excellent bake hardenability can be produced using a steel containing, in percent by mass, 0.2% or less of carbon, 3.0% or less of manganese, 0.0030% to 0.0180% of nitrogen, 0.5% to 0.9% of chromium, and 0.020% or less of aluminum by adjusting the ratio of chromium to nitrogen to 25 or more and the area ratio of ferrite to 80% or more.
Japanese Unexamined Patent Application Publication No. 2009-35816 discloses a method for manufacturing a high strength cold rolled steel sheet having low yield stress and little variation in material properties with annealing temperature using a steel containing, in percent by mass, more than 0.01% to less than 0.08% of carbon, 0.8% to less than 1.7% of manganese, and more than 0.4% to 2% of chromium by adjusting the composition ratio of chromium to manganese to Cr/Mn≧0.34 and the heating rate in annealing to lower than 3° C./s.
Japanese Unexamined Patent Application Publication No. 2006-233294 discloses a method for producing a steel sheet having excellent bake hardenability using a steel containing, in percent by mass, 0.01% to less than 0.040% of carbon, 0.3% to 1.6% of manganese, 0.5% or less of chromium, and 0.5% or less of molybdenum by cooling the steel to a temperature of 550° C. to 750° C. at a cooling rate of 3° C./s to 20° C./s after annealing and then to a temperature of 200° C. or lower at a cooling rate of 100° C./s or higher.
However, the steel sheet disclosed in JP '619 is difficult to use for outer panels because a large amount of silicon needs to be added to form retained γ, thus degrading surface quality. To form retained γ, additionally, the steel sheet needs to be maintained in the temperature range of 350° C. to 500° C. for an extended period of time. This results in formation of a large amount of bainite which noticeably increases YP and therefore degrades surface distortion resistance, thus making it impossible to use the steel sheet as an outer panel.
The steel sheets disclosed in JP '405, JP '350, JP '036 and JP '816 above, on the other hand, are dual-phase steels having a microstructure composed mainly of ferrite and martensite formed by controlling the composition thereof such as the manganese, chromium, or molybdenum content, to achieve low YP, high elongation, and high BH.
However, it has been demonstrated that, of the steel sheets disclosed in JP '619, JP '405, JP '350, JP '036 and JP '816 above, those containing a large amount of chromium have low yield stress and little variation in material properties, whereas those containing a relatively small amount of chromium have high YP and large variations in material properties.
That is, dual-phase steels having a hard second phase such as martensite as a strengthening structure essentially tend to have variations in material properties as compared to conventional solid solution strengthened steels strengthened with manganese or phosphorus. For example, the volume fraction of the second phase varies noticeably with variations of several tens of ppm in the carbon content of the steel or variations of 20° C. to 50° C. in annealing temperature, and the material properties tend to vary with variation in second phase fraction. This makes it difficult to sufficiently reduce surface distortion of a dual-phase steel sheet.
It has also turned out that it is difficult to form uniform and fine conversion crystals on steels containing large amounts of chromium, molybdenum, and silicon after conversion treatment, where numerous voids where no conversion crystal is deposited (regions where no crystal is deposited after conversion treatment) are found, meaning that they have insufficient conversion treatment properties.
In addition, as a result of detailed research on the corrosion resistance of steel sheets containing a large amount of chromium in actual parts, we found that these steels have insufficient corrosion resistance at a hem of a hood or door or at a spot weld and that the perforation life of a steel decreases by about 1 year if 0.40% of chromium is added thereto and decreases by 2.5 years if 0.60% of chromium is added thereto. That is, while chromium is conventionally believed to have the effect of slightly improving the corrosion resistance in a flat panel atmospheric exposure environment, it has turned out that chromium noticeably degrades the corrosion resistance in an environment such as at stacked portions of steel sheets where the steel is exposed to a wet atmosphere for an extended period of time and a corrosion product accumulates easily, thus requiring the chromium content of steel sheets to be significantly reduced for such applications.
The technique disclosed in JP '294 is difficult to apply without water cooling equipment or air/water cooling equipment because it requires rapid cooling at 100° C./s or higher after annealing, and a sheet subjected to water cooling or air/water cooling cannot be used as an outer panel because the flatness decreases noticeably.
Thus, no dual-phase or multiphase steel has so far been provided that has a low YP comparable to the current level and excellent stability of mechanical properties, corrosion resistance, and conversion treatment properties, and there is a strong need for a steel combining these properties among automobile manufacturers.
Accordingly, it could be helpful to provide a high strength cold rolled steel sheet that solves the above problem and a method for manufacturing such a steel sheet.